The present invention relates to a lithium-manganese compound oxide having a spinel structure having reduced lattice defects. The inventive compound oxide is useful as a cathode electroactive material in a lithium battery having a large initial capacity and which exhibits a small reduction in capacity even when repeatedly charged and discharged. The present invention further relates to a production method thereof, and a nonaqueous second battery using the inventive compound oxide as a cathode electroactive material.
As a cathode electroactive material of a nonaqueous secondary battery, particularly of a high energy density-type nonaqueous secondary battery, LiCoO2, LiNiO2 and/or a spinel structure compound oxide comprising lithium, manganese and oxygen have been proposed. LiCoO2 is disadvantageous in that it is expensive and there is a restriction in obtaining the raw materials therefor, and LiNiO2 is disadvantageous in that stable production is difficult. Accordingly, there is a need to develop a lithium-manganese spinel structure compound oxide (hereinafter referred to as a manganese-base cathode electroactive material) which is inexpensive yet provides good performance.
When a nonaqueous secondary battery using a manganese-base cathode electroactive material is repeatedly charged and discharged, the capacity is disadvantageously greatly reduced after a small number of charging and discharging cycles. Furthermore, there is a problem in that the practical capacity of the nonaqueous secondary battery is considerably lower than its theoretical capacity based on the composition of the manganese-base cathode electroactive material.
In order to solve these problems, a manganese-base cathode electroactive material having a composition where lithium is added in excess has been proposed, and a battery including this manganese-base cathode electroactive material exhibits a smaller reduction in battery capacity as compared with the initial battery capacity even when charging and discharging are repeated. However, this technique is deficient in that because lithium is added in excess to the manganese-base cathode electroactive material, the initial capacity is reduced as compared with conventional batteries, although the reduction ratio is small (see, JP-A-2-270268 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d)).
A cathode electroactive material for nonaqueous secondary batteries having a large initial capacity is obtained by synthesizing a manganese-base electroactive material at a low temperature. However, this manganese-base cathode electroactive material has low crystallinity and a large specific surface area, and therefore is disadvantageous in that although the initial capacity is large, the capacity is greatly reduced upon repeated charging and discharging (see, British Patent Publication No. 2,221,213A).
It has also been proposed to obtain a manganese-base cathode electroactive material from manganese dioxide having a specific surface area of 80 m2/g or more as a manganese compound, and to use this as a cathode electroactive material for a nonaqueous secondary battery (see, JP-A-6-275276). The manganese-base cathode electroactive material obtained by this method, however, has a true density of only about 4.00 g/cm3 and therefore, a nonaqueous secondary battery having a large initial capacity cannot be obtained. Also, the capacity is greatly reduced after a small number of charging and discharging cycles. Accordingly, this material is not yet suitable as a cathode electroactive material of a battery for practical use.
It is therefore an object of the present invention to develop a cathode electroactive material for use in a nonaqueous secondary battery including an anode comprising lithium, a lithium alloy or a material capable of doping or undoping lithium and a nonaqueous electrolyte containing a lithium salt, which can provide an initial capacity close to its theoretical capacity based on the composition of the cathode electroactive material and which exhibits a small reduction in capacity even when charging and discharging cycles are repeated many times. It is furthermore an objective of the present invention to develop a production method thereof and a nonaqueous secondary battery using this cathode electroactive material.
The above objects of the present invention have been attained by providing:
(1) a cathode electroactive material comprising a compound oxide having a spinel structure, said compound oxide comprising lithium, manganese and oxygen and having an atomic ratio of lithium to manganese of Li/Mn=0.48 to 0.55, a true density of 4.05 g/cm3 or more and a lattice constant of 8.240 xc3x85 or less;
(2) a cathode electroactive material comprising a compound oxide having a spinel structure, said compound oxide comprising lithium, manganese and oxygen and having an atomic ratio of lithium to manganese of Li/Mn=0.51 to 0.55, a true density of 4.05 g/cm3 or more and a lattice constant of 8.240 xc3x85 or less;
(3) a method for producing a cathode electroactive material comprising a compound oxide having a spinel structure, said compound oxide comprising lithium, manganese and oxygen and having an atomic ratio of lithium to manganese of Li/Mn=0.48 to 0.55, a true density of 4.05 g/cm3 or more and a lattice constant of 8.240 xc3x85 or less, which comprises mixing a lithium compound with manganese carbonate having a specific surface area of 10 m2/g or more, reacting the mixture at a temperature of from 350 to 680xc2x0 C. for at least one hour, and heat-treating the resulting spinel structure compound oxide at a temperature of from 730 to 900xc2x0 C.;
(4) a method for producing a cathode electroactive material comprising a compound oxide having a spinel structure, said compound oxide comprising lithium, manganese and oxygen and having an atomic ratio of lithium to manganese of Li/Mn=0.51 to 0.55, a true density of 4.05 g/cm3 or more and a lattice constant of 8.240 xc3x85 or less, which comprises mixing a lithium compound with manganese carbonate having a specific surface area of 10 m2/g or more, reacting the mixture at a temperature of from 350 to 680xc2x0 C. for at least one hour, and heat-treating the resulting spinel structure compound oxide at a temperature of from 730 to 900xc2x0 C.;
(5) the method for producing a cathode electroactive material as described in (3) or (4) above, which comprises reacting said mixture of a lithium compound and manganese carbonate at a temperature of from 500 to 650xc2x0 C. for from 2 to 40 hours and heat-treating the resulting spinel structure compound oxide at a temperature of from 750 to 850xc2x0 C. for from 5 to 30 hours;
(6) a nonaqueous secondary battery comprising the cathode electroactive material described in (1) or (2) above; and
(7) a nonaqueous secondary battery comprising the cathode electroactive material as produced by the production method described in any one of (3) to (5) above.